The present invention relates to the configuration of an optical communication system in which a plurality of subscriber units share an optical transmission line and a method for operating the system, and also relates to a system extension such as an extension of the transmission distance and an increase in the number of subscribers to be accommodated.
As the need for the communication utilizing the broadband increases, with respect to the access line dedicated to users, a large-capacity access line using optical fibers has been substituting for the access technique based on the telephone line such as DSL (Digital Subscriber Line). At present, in view of costs for line construction and maintenance management, the PON (Passive Optical Network) system (hereinafter, sometimes simply referred to as PON, an optical passive network system or passive optical network system) has been used frequently and widely as the access network. The ITU-T (International Telecommunication Union Telecommunication Standardization Sector) has been proceeding with standardization (recommendation) of the PON, and then the G-PON (Gigabit capable PON) standardized by the ITU-T recommendation G.984.3, for example, has begun to be introduced to the access network since around 2006 in individual countries in the world.
The PON is a system in which between an office side apparatus (hereinafter referred to as an OLT (Optical Line Terminal) and each of a plurality of subscriber units (hereinafter referred to as ONUs (Optical Network Units), optical signals are branched and/or multiplexed by using optical fibers and an optical splitter so as to be transmitted/received therebetween. Since the performance such as attenuation amounts of the optical signals passing through the optical fibers is limited by the transmission performance of the used optical fiber and the number of optical branching paths in the used optical splitter, the communication distance between OLT and ONU is limited. Taking a specified example, a GPON is used for which the communication distance is a maximum of 20 km and the number of branching paths (the number of ONUs connectable to the OLT) of the optical splitter is set to a maximum of 64.
As the opportunity of accessing the Internet and making communication by the home subscribers (communication network users) in order to collect information and keep social life increases, equipments of communication network, especially, access networks for connecting the subscribers to the communication network have been demanded to be increased. In other words, the carrier offering the communication network is urged to increase the capital necessary for increasing the number of subscribers accommodated by each station as the number of users of the access line increases. In order to increase the number of users, a method is conceivable which additionally introduces the PON per se used in the access network, that is, adds the OLT, or extends the number of ONUs which the OLT of PON accommodates. In the generally accepted configuration of the PON, however, the OLT totally carries out the control of a complicated system such as bandwidth control and the management of all of the ONUs accommodated and is far more expensive than the ONU. The costs for newly constructing optical fibers also incur a great expense imposed on the carrier. Accordingly, as the method for solving, increasing the number of ONUs accommodated per OLT is preferable to adding OLTs.
In connection with the existing PON, the study of a relay unit used for communication distance extension and increasing the number of branching optical paths (hereinafter referred to as an EB (Extender Box)) has been starting. In basic concept thereof, the EB is installed appropriately inside an optical signal communication section between the OLT and the ONU, and is controlled by the OLT to realize extending the communication distance of the optical fiber and increasing the number of optical branching path. Then, for the control protocol, standardization by the ITU-T proceeds at present which is based on a proposal purporting that the OMCI (ONU Management Control Interface) representing the existing ONU control protocol is used. This makes it possible to offer high-speed Internet access service to a region for which spread of IT technology is more behind the times than for the urban central region, and is noticed as one of methods for spreading the access network (ITU-T recommendation (proposal) G. 984, re).